The present application is the national stage under 35 U.S.C. 371 of PCT/SE99/00122, filed Jan. 29, 1999.
This invention relates to a cylindrical screen for separating undesired particles from a fibre suspension, namely a screen which has an inner side and an outer side and which is provided with a plurality of circumferentially extending projections on the outer side of the screen, grooves formed on the inner side of the screen and extending in a direction having an axial component, and screening passages in the form of slots disposed in the area of each groove and diverging from the inner side of the screen towards the outer side thereof.
A cylindrical screen of this kind may form part of a pressure screening apparatus of the kind used for cleaning of papermaking pulp. Such a pressure cleaning apparatus conventionally comprises a housing in which the screen is accommodated as a stationary part, and a rotor which is rotatably mounted coaxially inside the screen. A fibre suspension to be freed from undesired particles is fed into an annular space between the rotor and the surrounding cylindrical screen, and the greater part of the fibre suspension is forced through the screening passages of the screen, while the greater part of the undesired particles continues along the annular space and is discharged axially therefrom. Normally, the fibre suspension to be cleaned is fed into the annular space at one axial end of the screen and the undesired particles are discharged at the other axial end of the screen. Within the housing, the screen is surrounded by a compartment in which the cleaned fibre suspension, the accept part, is collected and conveyed to an outlet. At the said other axial end the housing has an outlet for the undesired particles, the reject part.
As stated initially, the screening passages, commonly in the form of elongate slots, diverge from the inner side of the screen towards the outer side thereof. The screening passages may be given that form in connection with cutting them in a planar plate which is then formed into a cylindrical screen by bending. Upon bending of the planar plate into cylindrical shape, the width of the slots is reduced at the inner side of the screen. The more the screening passages diverge, the lesser the risk of the fibres in the fibre suspension becoming stuck in the screening passages and therefore the greater the accept flow through the screen. In order that the best possible cleaning of certain kinds of fibre suspension, such as papermaking pulp containing recycled fibre, may be achieved, the width of the screening passages should not exceed about 0.1 mm at the inner side of the screen.
WO93/04797 discloses a cylindrical screen manufactured as follows: One side of a planar plate is formed with a large number of parallel V-grooves, and backing ribs extending transversely to the grooves are welded to the raised portions separating the grooves. The other side of the plate is also formed with grooves, which are located opposite to the grooves on the first side of the plate. In the bottom of each groove elongate screening passages are formed by milling from the said other side of the plate such that the plate is divided into a multiplicity of bar-like elements held together by the backing ribs. Then the plate and the backing ribs are bent to form a cylindrical screen with the backing ribs extending circumferentially on the outer side of the screen. The bending operation results in a reduction of the width of the screening passages at the inner side of the screen.
The screen disclosed in WO93/04797 has the following disadvantages:
Firstly, it is not possible to provide screening passages which diverge adequately from the inner side of the screen towards the outer side thereof, because the screening passages are milled from the said other side of the plate so that the screening passages have to have parallel longitudinal walls or even have to be wider at the said second side than at the first side. only when the plate is bent to cylindrical shape will the screening passages diverge slightly from the inner side of the screen towards the outer side thereof.
Secondly, it is very difficult to make the screening passages uniform. The welding of the backing ribs to the raised portions separating the grooves results in stresses in the welds. When the screening passages are milled subsequent to the welding, the stresses will cause variations of the width of the screening passages along the length thereof. Moreover, the welds have to be exactly alike in order that they may produce a uniform bending resistance. Even very small differences between the welds will result in the width of the screening passages varying still more as a result of the bending of the plate into cylindrical shape. Uniform screening passages are a prerequisite for a proper separation of undesired particles from fibre suspensions. Because the screening openings are only one or a few tenths of a millimetre wide, dimensional variations as small as 0.01 mm will have a negative impact on the quality of the separation.
Thirdly, when using a plate of a thickness that is economically acceptable having regard to the screen diameter in each particular case, it is very difficult to mill screening passages narrower than 0.3 mm, and it accordingly is difficult to obtain, as a result of the bending of the plate, screening passages that are only 0.1 mm wide at the inner side of the screen.
The object of the present invention is to provide a cylindrical screen of the kind indicated initially and having uniform screen passages which diverge adequately from the inner side of the screen towards the outer side thereof and which can readily be made with a width of only about 0.1 mm. This object is achieved in accordance with the invention by producing the aforesaid projections, grooves and slots in a screen of the kind defined by working of a substantially planar screen plate bent into cylindrical shape subsequent to the working.
By producing the projections, grooves and slots of the cylindrical screen by working of a planar plate, welding operations in the initial steps of the manufacture of the screen are avoided. Thereby it is possible to achieve uniformity in the width of the screen passages. If the projections are formed by milling, for example, a high dimensional accuracy is achieved, and the projections will be uniform and have a uniform bending resistance. As a result, the screening passages will remain uniform after the bending of the plate. Alternatively, the projections may be formed by rolling of the planar plate. Using a modern production method, such as laser beam machining, makes it possible to shape the screen passages such that they diverge even in the planar plate, that is, before the bending operation is carried out, and the width of their narrow end can be made as small as about 0.22 mm with an accuracy of about 0.01 mm. Owing to the projections, it will be possible to reduce the width of the screening passages at the inner side of the screen to about 0.1 mm as a result of bending the plate into cylindrical shape.
In one embodiment of the invention, the grooves and slots are produced by material-removing working of the screen plate.
In another embodiment of the invention the projections are also produced by material-removing working of the screen plate.
In the above-mentioned embodiments of the invention, it is not necessary for every projection to extend throughout the circumference of the screen. This is so because it is also possible to reduce the width of the screen passages using projections which extend only partly around the screen but overlap one another circumferentially.
However, in a preferred embodiment of the invention, at least one and preferably all of the projections extend throughout the circumference of the screen.
According to a further embodiment of the invention each projection has an outer face and the slots extend between adjacent projections, and the cross-section of the projections as viewed in a longitudinal section of the screen extends integrally from said outer face of the projection to the inner surface of the screen. As an advantageous result, the screen according to the invention is substantially stronger than a screen according to WO93/04797, because the screen according to the invention is solid and unitary throughout the thickness of the cylindrical plate, from the inner side to the outer side, in the area of the projections. At the same time, the projections produce the effect of reducing the width of the screening passages at the inner side of the screen upon bending the plate into cylindrical shape.
A particularly favourable effect as regards the reduction of the width of the screening passages at the inner side of the screen is obtained if the height of the projections is at least twice the thickness of the cylindrical screen plate in the area of a slot.
In a preferred embodiment of the invention, at least one of the riblike projections is wider adjacent its outer face than adjacent the region of the slots. In this embodiment, the cross-sectional surface of a projection as viewed in an axial section through the screen has its center of area at a substantial distance from the inner side of the screen, namely in the radially outer portion of the projection, and as a consequence the reduction of the width of the screening passages will be substantially amplified at the inner side of the screen upon bending of the plate into cylindrical shape. This effect is obtained even if the planar plate used as a starting material for the manufacture of the screen is relatively thin.
The distance between the projections of the screen should be in the range of 10 to 100 mm, preferably within the range of 20 to 40 mm. Moreover, the grooves of the screen on the inner side thereof and the slots should include an angle xcex1 in the range of 1 to 40xc2x0 with a generatrix of the cylindrical outer surface of the screen as indicated in FIG. 2. A screen having grooves thus skewed or oblique enables a more rapid transport of the reject part towards the reject outlet of the screen than does a screen the grooves of which extend exclusively axially and thus do not have any circumferential component.
In order that a good flow of the accept part through the screen may be obtained, the circumferential distance between adjacent slots should be less than 10 mm, preferably 5 mm.
If the screen has slots having a width at the inner side of the screen within the range of 0.05 to 0.2 mm, preferably within the range of 0.07 to 0.15 mm, the screen is very well suited for separating undesired particles from papermaking pulp containing recycled fibre.
The invention is not limited to a screen only having screening passages in the shape of straight slots. Thus, at least some of the screening passages may extend along a be wave shaped path. Owing to the projections, the width of wave shaped slots will also be reduced at the inner side of the screen upon bending the screen into cylindrical shape. However, if the wave shaped slots are formed by laser beam machining, it is easy to vary the width of each slot along the length thereof such that the width of the slot will be constant throughout the length of the slot after the planar screen plate has been bent into cylindrical shape.
The invention also relates to a method of manufacturing a cylindrical screen for separating undesired particles from a fibre suspension, the method including the following manufacturing steps:
forming on the first side of a planar plate having a first and a second side a plurality of parallel grooves by material-removing working of the plate,
forming slots in the planar plate in the area of each groove by cutting the plate, and
bending the plate into cylindrical shape such that the grooves will be on the inner side of the resulting cylindrical body.
The method according to the invention is characterised in that a number of projections extending in a direction substantially transverse to the grooves are formed on the second side of the planar plate by material-removing working of the plate prior to its bending into cylindrical shape.
In the method according to the invention, at least one and preferably all of the projections may be formed such that subsequent to the bending of the plate to cylindrical shape, the projections will extend throughout the circumference of the screen.
In the screen according to the invention the slots may advantageously be formed by laser beam or water jet machining.
Preferably, the slots are shaped such that they diverge from the first side of the planar plate towards the second side.
The invention will be described in greater detail below with reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic illustration of a pressure screening apparatus including a cylindrical screen according to the invention;
FIG. 2 is a more detailed illustration of the cylindrical screen according to the invention; and
FIGS. 3 to 6 show longitudinal sectional views, taken in an axial plane indicated by line Vxe2x80x94V in FIG. 4, of two embodiments of cylindrical screens according to the invention.